Enteric Redmouth, a bacterial disease of salmonid trout, can be a serious problem with intensively cultured species of fish and is one of the limiting factors in the culture of trout. The disease can occur at any time of the year, especially when water temperatures exceed 14.degree. C. Consequently, it is most prevalent in the warmer summer months. Outbreaks can be expected when the water temperatures reach 14.degree.-20.degree. C.
Enteric Redmouth is caused by the organism, Yersinia ruckeri. This bacterium is a gram-negative, motile rod, approximately 1.times.3 nm. The motile organisms can frequently be seen in wet slides of fresh kidney or spleen smears from infected fish (900.times.magnification). It is prevalent throughout North America, primarily in the major trout producing areas.
Vaccination against Enteric Redmouth is gaining popularity among aquaculturists as a prophylactic control method. However, the lack of a convenient efficacious delivery method applicable to mass immunization of fish has limited the use of vaccines for controlling Enteric Redmouth. Although several techniques are available for mass inoculation, for example, oral delivery, injection and forced fluid influx procedures such as vacuum infiltration, hyperosmotic infiltration and pressurized (generally about 90-100 p.s.i.) spray vaccination, each method, except injection, delivers unquantitative vaccine doses.
For example, incorporation of vaccine bacterins into the diet is a desirable delivery system because of its ease of administration. However, the degree and duration of immunity conferred is generally less than delivery by injection. This is perhaps due to the difficulty of ensuring uniform dosage uptake of the diet by the fish.
With regard to the injection technique, the inherent disadvantage is obvious. Although fish can be injected individually on a limited scale, the individual injection of millions of small fish is not practical.
The disadvantages of pressurized spray vaccination techniques are its inapplicability to small fish weighing less than four grams and the possible hypersensitization to the vaccine by the user due to the inherent aerosol mist produced by the technique. Forced fluid influx procedures such as vacuum infiltration and hyperosmotic infiltration possess a serious disadvantage in the traumatic physiological response of fish to the stress induced by these pressurized procedures. In addition, hyperosmotic infiltration procedures may require the additional step of pretreatment in a hyperosmotic solution of, for instance, urea, or NaCl, before hyperosmotic exposure to the vaccine solution.
Whereas, it had previously been thought that a pressurized procedure was necessary to affect vaccine absorption by fish, it has now been found that the mass inoculation of salmon and trout against Enteric Redmouth can be readily accomplished without the aforementioned disadvantages by simple non-pressurized immersion in a vaccine derived from Y. ruckeri.